Many modern wind turbine generators (WTGs) are equipped with generally long and slender blades that are designed to flex or bend in response to wind forces. While some bending is expected, the blade can suffer damage if it bends beyond an extreme limit or too frequently. The risk of such damage is greater in proportion to the ever increasing size of blades used in modern WTGs. Therefore, to detect the occurrence of bending, among other reasons, modern WTG blades are sometimes equipped with bending moment measuring sensors that measure a bending moment experienced at one or more points along the span of each blade. To avoid electromagnetic interference the sensors are often implemented using fiber bragg gratings (FBGs) in a fiber optic cable.
Bending moment sensors are not limited to use with blades of a WTG. Therefore, the use of bending moment sensors with WTG blades is merely one example use. Another example use includes measurement of a bending moment in a conduit, such as a marine riser used in deep water oil exploration, or any other structure having a generally long slender shape used in a manner and/or environment that exposes the structure to frequently changing loads and/or extreme loads. One problem associated with the use of any sensor is the risk of the sensor entering a fault mode. A fault mode is a mode in which the sensor behaves in a way that deviates from its nominal behavior. The nominal behavior is the one that fulfills all the requirements imposed by systems that directly or indirectly make use of the sensor's output. Thus, risk of failure or poor performance for any such systems will increase if the sensor enters a fault mode. Accordingly, identification of the occurrence of a fault mode can help reduce risk of failure and improve performance. Moreover, reducing risk of failure and poor performance is useful for modern WTGs because they operate in remote locations, making technician servicing difficult and costly, and are often required to have long operational lifetimes, e.g., twenty years.
U.S. Patent Application Publication Number 2010/0232961 (the '961 patent publication) describes one fiber optic sensor that is used to measure a bending moment experienced by blade of a WTG. The optical fiber used in the '961 patent publication has two alternative output points, each connectable to a data processing device. Consequently, in the event of a breakage in the optical fiber, signals from the sensor are available from at least one of the output points.
The '961 patent publication appears to address the occurrence of only one fault mode, namely, optical fiber breakage. Other sensor fault modes may occur, however, such as partial or complete loss of power in the sensor output, loss of sensor calibration, and/or signal processing errors. Moreover, fault modes of a sensor may manifest at a high level (i.e., at a level that takes into account external data or data other than that available from the sensor) or at a low level (i.e., at a level that takes into account internal data or data available from or pertaining to only the sensor). Identifying other fault modes would help increase reliable performance and reduce the risk of failure and associated repair costs. Furthermore, the ability to distinguish among different fault modes and fault modes at different levels would improve the ability to identify appropriate diagnostic techniques to apply.